Control system for electric steam generators



Sept. 30, 1952 H. c. MITTENDORF ET AL 2,612,593

CONTROL SYSTEM FOR ELECTRIC STEAM GENERATORS Filed July 25, 1947 4 Sheet s-Sheet l INVENTORS HARVEY C. MITTENDORF R J N o w L L M M.JA w MW D WW FIG. I

Sept. 30, 1952 H. c. MITTENDORF ET AL 2,612,593

I CONTROL SYSTEM FOR ELECTRIC STEAM GENERATORS Filed July' 25, 1947 4 Sheets-Sheet 2 54 l3 7 BLOW INVENTORS DOWN AND HARVEY C.MITTENDORF BY WlLLlAM 1.. PAULISON JR.

FIG. 3

Sept. 30, 1952 H. c. MITTEN DORF ET AL 2,612,593

CONTROL SYSTEM FOR ELECTRIC STEAM GENERATORS Filed July 25, 1947 4 Sheets-Sheet 4 IMMERSION 63 l 2 MAX. .l RI

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ARE NORMAL INVENTORS I AND HARVEY c. MITTENDORF WILLIAM L. PAULISON JR. FIG. 5 BY I ATTO EY Patented Sept. 30, 1952 UNITED- STATES Ti 7 s nce CONTROL SYSTEM FOR ELECTRIC STEAM GENERATORS Harvey CrMittendorf, East Orange, and William L. Paulison,. J r., Ridgewood, N. J.; said Mittendorf assignor to Combustion Engineering- Superlieater, 11:10., a corporation of Delaware, and said Paulison assignor to Bailey Meter Company, a corporation of Delaware Appiica'tionduly 25, 1947, SerialNo. 763,700

34 Claims.

This invention relates to control systems and particularly to method and apparatus adapted to control the operation of a power producing apparatus such as an electrically heated vapor generator.

In certain localities where hydro-power is abundant, the price of electricity is low. enough to make economical its use in vaporizing water into steam for process or other purposes. In the operation of such vapor generators problems of control arise which must be solved in order that the temperature and pressure of the produced steam will be of uniform optimum value under existing demand conditions and with ,full safety to life and apparatus.

It is an object of our invention to provide both method and apparatus of control for an electricallyheated vapor generator.

We have as a further object the provision of a completely automatic control system for such a vapor generator. I

Other objects will become evident upon a study of the specification, drawings and claims.

In the drawings:

Fig. 1 is a diagrammatic representation of one preferred embodiment of ourinvention applied to an electrically heated steam generator- Fig. 2 diagrammatically illustrates a pneumatic control system for regulating .the rate of supply of feed water to the boiler of Fig. 1.

Fig-3 diagrammatically illustrates a pneumatic control for regulating the blow-down from the boiler of Fig. 1. v

Fig. 4 is a chart of conditions in the operation .of the unit of Fig. 1.

Fig. 5 is an across-the-line diagram of electrical circuits for actuatingcertain valves of Fig. l to accomplish the sequential operation charted inFig.4. Q

Referring to the drawings, the particular. unit being described and to which our control is applied, includesa vertically mounted drum or pressure vessel l supported in customary manner (not shown). Insulated through the upper head of the drum 1 and suspended therefrom are three equally spaced electrodes 2 (twoonly being shown in Fig. 1), which maybe of cast iron or other suitable material. The three electrodes are usually enclosed by a clover leaf neutral plate properly spacedfrom the electrodes and from the inner surface'of the drum wall. The terminals3 of the electrodes are connected through water itself.

the necessary electrical apparatus to also'urce of electrical power. (usually 3p hase); the..ar rangement forming no part of the present inven- External to and above the drum l' isahorizontally supported dry drum i having-a pplurality of steam riser tubes 5 and one or more dry drum'drain tubes 6. Known bafile arrangements 'l'are incorporated within the dry drumf i to separate from the steam, any-waterwhich may be carried over or condensedl The boiler druml is provided'with theusual water column and 'gage glasses (not shown),'whilethe dry drum d' 'is provided with the usual safety valvei'not shown) and with a main steam exit nozzle 8 discharging to a steam header 9. Water is" suppIiedtothe unit through a feed pipe Id 1 terminating irl -a feed nozzle i I centrally located in thelowerin'o'st portion of the main drum l. To as'ettlingcha'mber portion i2 at the base of the drum I is also connected a blowdown line I3 and a bleed man.

The problems with which our methodandiapparatus are concerned is the supply of watervto the unit, the extraction of Water 'therefrom,1 the maintenance of desired level of liquid within the drum 5, as well as the conductivity 'of 'the 'liquid within the drum; all in proper degree sothat the vapor which is generated will satisfy the rate demand while maintaining the ==desired pressure.

In such a vapor generator, the heat for vaporizing the water is supplied by the passagefof-electric current through the water itself, fromone electrode to another, or 'tothe neutral-plate. Since the voltage of the power supply isconstant, the energy consumed varies directly with the current, While the current varies inversely as the resistance of the path it travels. sistance of the path, in tu.rn,-depends upon the cross-sectional area of "the water conductor (path) and upon the specific resistance of the fixed in position, the water-area through. which the current can pass. is determined by the depth of immersion of the electrodes; i. e. the height I of-water within the boiler drum. The specificresistance of the water is a function of thec'oncentration or amount of dissolved solids in the water and also the temperature of the .waterh f.

. Thus, in a boiler of this type, the amount; of steam generated depends upon the'amount of electric current passing through the water and The re-' Inasmuch as the electrodesare this, in turn, depends upon the area of the path through the water and upon the conductivity (or resistivity) of the water to the passage of electric current therethrough. The water area through which the current can pass is determined by the depth of immersion of the electrodes. Therefore, if we maintain a constant specific resistance of the water, the amount of steam generated may be controlled by varying the level of the water in the boiler.

Thus, in order to maintain a substantially uniform value of steam pressure, while satisfying the demand upon the boiler for quantity rate of steam generated, we are confronted with the problem of controlling the level of liquid within the boiler drum as well as the specific resistance of the water. A variation in any one, or all, of

the three variables, namely, the amount of water within the boiler, its concentration of impurities, or its temperature, may cause a desirable oran undesirable variation in steam pressure and/or rate of steam generation.

The water fed to the unit will always contain :a certain percentage of dissolved or suspended solids, and particularly if the greater portion of water supplied is other than condensate; Inasmuch as the steam does not carry away any appreciable amount of salts or solids from the boiler, there is a continuous tendency to build up the concentration of solids within the boiler water, and thus to continually decrease the re- -sistivity to current passage through the water path. When operating at rated capacity the boiler may hourly evaporate many times the water normally contained in it. In general, it

-may be said that if the conductivity of the water is maintained substantially uniform the rate of steam generation will vary with the water level or extent of immersion of the electrodes in the water pool. Conversely, if the level is maintained substantially uniform, then the rate of steam generation will vary with the concentration of dissolved salts in the boiler water. In order to keep the concentration of the water from gradually increasing (which would make inaccurate any functional relationship between water level and vapor generation) it is advisable to continually bleed some part of the highly concentrated water from the lower portion of the boiler drum in a controlled manner. If this is properly accomplished then a control of level within the drum is a control of rate of steam generation.

The interrelation of the effects of the operating variables may, however, cause disturbances in the primary control of level or of conductivity. For example, the conductivity of the water is not only a function of the amount of dissolved solids therein, but is also a function of the temperature of the water (increasing with temperature).

'By way of example; if there is an increase in demand upon'the vapor generator the steam pressure will tend to fall. Assuming for the moment a uniform conductivity of the water, then 'the increase in demand indicates a desirability of raising the level of water within the boiler, and thus increasing the path for electric current, thereby increasing the rate of current dissipation and consequently the rate of steam generation. However, a rapid increase in the rate of supply of water will tend to dilute the water within the boiler, thus lowering its conductivity, and at the 1 same time will tend to cool the water within the boiler, which also effects a lowering of its conductivity. Both of these adverse effects are opposite in nature to a desired increase in vapor generation as would be expected through increasing the level and thus the area of conductor path. It is, therefore, apparent that a proper control must be judicious in regard to varying the level of water and must definitely take into account such adverse effects as have been mentioned.

It is known to provide a generating unit of this type with a storage or surge tank to which water may be transferred from the boiler, or from which water may be fed to the boiler. Such a surge tank I5 is shown in Fig. 1 as being of substantially the same size as the dry drum 4 and supported above the boiler I in a similar manner. Preferably the surge tank I5 is a cylindrical pressure vessel, supported horizontally, to minimize changes in static head due to level changes when water is transferred to or from the tank. The horizontal length of the tank is chosen in accordance with the water storage capacity desired, in relation to the size of the boiler I, its steaming capacity, and the extent and rapidity of demand fluctuations expected. Certain features of the surge tank I5 and of the heat exchanger IB are disclosed and claimed in Patent No. 2,485,762 to Harvey C. Mittendorf.

Water within the tank I5 is transferred thereto from the boiler I, through a pipe I6, the pipe I6 communicating with the boiler I, at-a location such that the water transferred to the tank I5 is of a temperature and conductivity fairly represent-ative of the water in the pool in which the electrodes are immersed, ire. shielded by a baffle I! from the relatively colder and lower concentration feed water being admitted through the nozzle I I and not from the settling chamber section I2.

While the water stored in the tank I5 is at substantially the same pressure and temperature as the water in the boiler proper, there is of course some heat loss due to radiation. In order to conserve the heat of the highly concentrated bleed water controllably discharged through the pipe I 4, and to maintain the water within the tank I5 at or near boiler water temperature as possible, we pass the bleed through a heat exchange coil I8 located in the tank I5.

Water is transferred between the boiler l and the surge tank I5 through the pipe Ili under the control of a relatively large solenoid actuated valve C and a relatively small solenoid actuated valve D, the latter by-passing the valve C.

trical circuits in Fig. 5, while in Fig. 4 we have tabulated the open or closed positions of valves A-BCD for various operating conditions.

In the operation of an electrically heated vapor generator, and particularly one which is subjected to relatively rapid and wide load demands, a study of the relation between concentration of salts or solids in the water and the electrical current dissipation has shown that concentration has a very important bearing on the performance of the unit. The purpose of theisurge tank is primarily to assist in holding the concentration 5 of the boiler water uniform during load changes,

"against theadverse effect of dilution or the boiler --poe1 b-y feed water of low concntrationandtemperature. For the surge tank to be "useful in handling load-swings its water and heat storage must be of" sufficient magnitude to nearly satisfy the load swing vaporization.

"The continual evaporation oi waterih thedrum 'l tends to increase the. concentration; of the salts and impurities in the water pool in which the electrodes are immersed. In order" that steaming rate may be primarily controlled by-the amount of electrode immersion, it i's essential thatthe concentration valueof the pool be maintained as .nearlyuniform as possible. "water to (on decreasing steam demands) 'and' from.i('on increasing steam demands) 'the surge "tank'serves tominimize theconcentration dilution and'temperature quenching eiiect of sudden *drastidincreasesin rate of freshwater infeed.

The transfer of We provide additionally a check back irom'a "measure of concentration or conductivity of the pool-water, byregulating a bleedof highly concentrated water from a measure of conductivity.

In- Fig. l we'have-s'h'own-the bleed pipe- M connected to the settling chamber 12. Located in the pipe 14 is a regulating valve Zllunder the control of aconductivity controller ill responsive to-a conductivity cell'22', which is sensitive-to the conductivity of the pool water at-a selected location in the drum I. 'We haveiound that, particularly on rapidly and widely swinging steam demands, the-conductivity control of bleed is not alone suflicient, due to instrument time lag and also because it is not practical to sample from the conducting path directly from'the active'area between the electrodes themselves. Under such operating conditions we preferably employ both "a controlled bleed of" highly corrcentrated water and the automatic transfer of water-between the boiler and the surge tank as described. I

The primary functions ofour controlsystem may be summarized as follows: Y

'1. To so control the rate ofadmission offe'ed water totheunit as to maintain a-uniformsteam pressure, through varying the liquid immersion of theelectrodes in relation to steam demand. We preferably control the rateof admission of "feed waterconjointly from steam pressure, steam flow rate, water flow rate, and water'level within "the-boiler; with the demand factors ,(steam pressureand steam fiowrate) so limited'in their dictates as to prevent exceeding the desiredhigh and low immersion of the electrodes,

2. To controllably bleed highly concentrated Water from the unit under the dictates of a conductivity sensitive device and to transfer water between the pool and a storage tank under. chang- 'i'ng'load demands, all'for the purpose of maintaining the conductivity of the water in which the electrodes are immersed substantially constant at all levels and rates of operation, to the end that a control of extent of immersionbecomes a control of vaporization.

3'. To prevent abnormal high water:l'evel in the boiler by an emergency blowdown.

Giving consideration to suchdisturbing influshoes as have been mentioned, which'are effective principallyunder conditions of wide-and rapid '-variations in demand, the arrangement of Fig. 1 is particularly directed to most efficient operation of a unit subjected to' varyingload demands,

although not limited thereto.

- Theoretically,; with. a constant voltage applied to: the-electrodes. and. a uniform conductivity of {the-boiler. water,;the rate of steam generation "immersion ofthe electrodesfi.

in the relationship between "steam output.

will vary 'directly witnmh'e extent of immersion (if the "electrodes; 'i. i e. the leveljof water in the boiler.

Thus we desirea. geared range between steam pressure (asan indication of demand upon the boiler) and water level, namely, ade'fi'nite relation between level and load demand.

Connected to the steam outflow conduit! we show- :a 'Bourdon tube '23 "sensitive to pressure of the steam and arranged to vertically position the stem 24, of a pneumatic pilot valve 25, which is adapted to establish inthe pipe 26' an air loading pressurebearing a definite relation to the pressure of the steam in'the conduits. Such a pilot va'lve is described in the patentto Johnson -2;054,464. "We further provide a measuring dev iceil-arranged to vertically position the stem 28, oi'a pilot valve 29, toestablish in'the pipe 30 an-air loading pressure continuously representativeof the weight rate of 'flow of steam leaving the dry drum 4 through the conduit 9.

In connection with the feed water supply conduit 18 we show a water flow measuring device 3| arranged to vertically"position the stem32, 'ofa pneumatic pilot valve 33','thereby-continually establishing in'the pipe 34 an air loading pressure directly. representa'tive'of' weight rate of flow of feed water through the pipel fl to the boiler l.

"Spanning the vertical elevation ofthe'boiler l at predetermined maximum-andminimum water level" limits, are pipe connections 35-36, con-- nected to a Water level measuring device 31,

arranged to vertically position'the stern 38, of a pneumatic pilot valve 39, continuously establishingin the pipe ii) an air loading pressure representativeof the level of liquid within the drum-i between the points of connection ofpipes 35-46, and thus continually representative of the liquid In similar fashion we indicate at Al alevel measuring device spanning the elevation of the tank I 5.

Referring now to Fig. 2, it willbe' seen that the rate of'supply of water to the dru'm l, through the pipe I0, is controlled by a valve 42 which is positioned under the dictates of four variables in the operation of the unit. It is apparent that under any steady load'conditioh the rate of" li'quidinflow' to the boiler should equal the weight rate of vapor outflow plus the bleed. Therefor" the primary demandor load factor will be from steam outflow. This primary control factor may then be readjusted from-steam pressure so as to quickly anticipate load changes. As steam pressure decreases (indicating increased demand or steam outflow) the rate of water supplied to the boilerwill' be increased proportionately to the deviation "of steam pressure from standard. The electrodes are subject to wearing-away or scaling up, with consequent variation insteam generation rate per inch of submergenc'e. Steam pressure as an element'in the control of feed watersupply has the advantageof correcting forsuch changes leveland rate of Should there be fluctuations in feed water pressure, the feed water flow will fluctuate accordingly, so that a measure of water'level'is used to modify the primary demand effect so as tomaintain the level at a'proper value. "This control so iar described tends to maint'ain a definite water level for a given load demand.

There: will. however, be little. effect upon the valve 42: fromtthexsteam pressure: controller or from. the'vvater level controller inasmuch as under such steady load condition thesenvariable's p. s. 1., while the pressure in the should not be fluctuating. However, should any of the variables. tend to fluctuate they will impose a correction upon the loading pressure within the pipe 43 leading to the valve 42 and correspondingly correct the rate of water input to the boiler.

As a final check-back we use a measure of actual weight rate of, water flow through the comparison between the two might indicate a proper proportionality between weight rate of liquid inflow and weight rate of vapor outflow, whereas actually the proportion would not be exactly correct. With the check back, however, from actual water level such discrepancy will be corrected. At the same time the modified control from steam pressure will be efiective to take care of minor departures in steam pressure from the desired value.

When a change in load (for example an increase in steam demand) occurs, this will show up as an increase in the steam flow and a corresponding decrease in steam pressure. As previously mentioned, the desired operation is to raise the water level on the electrodes to increase the rate of steam generation. The in- :creased steam flow and the decreased steam pressure will act in the same direction to demand an increase in rate of feed water until the comparison between steam outflow rate and feed water input rate attains desired proportionality. If the water level has not reached the proper value to satisfy the new demand, the water level controller will impose its control upon the feed water valve 42. In general, the four variables,

namely, steam outflow, water inflow, steam pressure and water level, will coact to properly position the feed input valve 42 to satisfy the steam demand upon the unit and maintain steam pressure substantially uniform.

It will be appreciated that the various control ,instrumentalities, such as regulators, relays, pilot valves, etc., may be adjusted for sensitivity, range, limits, etc. Inasmuch as such adjustments are known, it does not appear to be necessary to go into the details thereof.

It may be said that in Fig. 2 an air loading pressure is established within the pipe 26 representative of steam pressure, an air loading pressure within the pipe 30 representative of weight rate of steam outflow, an air loading pressure within the pipe 40 representative of water level within the boiler drum 1, while an air loading pressure is established in the pipe 34 representative of weight rate of water feeding the boiler.

We apply the steam flow effect to the A chamber of a relay 44, to the B chamber of which we apply the steam pressure effect. Connected to the inlet and outlet valves of the relay '44 are lead limiting relays 45, 46 supplied with air under pressure from a source (not shown). The relays 45, 46 supply air at selected reduced pressures to the pipes 41, 48 respectively, joining the D chamber of relay 44. By way of example, the pressure in the pipe 41 may be limited to 25 pipe 48 may be limited to 5 p. s. i. 5 i 1 Variations in the loading pressures, effective in the chambers A and B, produce a control pressure in the chamber D to which is connected the pipe 49. The C chamber is open to the atmosphere. The result is that for any change in the loading pressures applied to the A chamber or to the B chamber the control pressure in chamber D will vary proportionately and in proper direction.

We .desirably limit the water level in the boiler drum to a maximum and a minimum value irrespective of steam pressure conditions. When the loading pressure from the relay 44 (in pipe 49) reaches either a minimum or a maximum value, as determined by the adjustment of the limiting relays 45, 46, no further change in loading pressure from the relay 44 can be applied to the relay 50. Under such conditions the water level controller 31, connected to the B chamber of relay 50, assumes precedence over the demand effect of steam pressure and steam flow, and maintains the water level in the boiler drum within the desired limits without regard to steam pressure or steam flow changes. Thus the boiler is prevented from flooding or from becoming empty.

The limiting relays 45, 46 are each provided with an adjustable orifice bleeding to the atmosphere so as to definitely establish the maximum pressure which may be experienced in the lines 41, 48 respectively. In order to limit the maxi- .mum and minimum values of the control pressure available in the pipe 49 we limit the maximum air pressure in the pipes 41, 48 and thereby in the D chamber of relay 44. Assuming that a maximum pressure of 25 p. s. 1. is available in the pipe 41, through adjustment of the bleed valve of the relay 45 and therefor to the inlet of the chamber D, then this is the maximum control pressure which can be applied to the pipe 49 when the inlet valve is open and the outlet valve of the relay 44 is closed. Under a reversed condition, when the inlet valve of relay 44 is closed and the outlet valve is open, the bleed valve of limiting relay 46 is so adjusted that the pressures within the chamber D of relay 44 cannot decrease below a minimum of say 5 p. s. i.

The loading pressure resultant from steam pressure and steam flow, available in the pipe 49, is applied to the A chamber of a relay 50 for comparison with the water level efi'ect which is available within the B chamber from the pipe 40. The output of the relay 50, available through a pipe 5|, is applied to the A chamber of a standardizing relay 52, to the B chamber of which we apply the water flow eflect through the pipe 34.

The output of the relay 52 is available in the pipe 43 for positioning the feed valve 42. Interposed in the pipe 43 we provide a selector valve 53 similar to the type described in the patent to Fitch 2,202,485, thus providing a possibility for remote manual or automatic selective control of the valve 42.

The relay 5'2 is of a standardizing type described in the patent to Gorrie, 2,098,914. Variations in pressure within the A chamber or the B chamber of the relay initially vary the control pressure within the pipe 43 with a standardizing follow-up variation in such control pressure accomplished through the agency of the adjustable bleed between the D and C chambers of the relay as fully described insaid Gorrie patent.

It will be evident from a study of Fig. 2 that the differential relay 44 continually interrelates pressures individually representative of the two operation variables first to show the effect of changes. in demand upon the boiler, namely;-

steam pressure-and steam outflow rate. Either or both of'these operation variables may change, but normallythey vary in opposite. direction, i. e. an increase. in steam outflow rate is accompanied by a decrease in steam pressure, and vice versa. Thus, upon an increase in steam outflow, thexpressure within the A chamber of relay 44 will increase, while the pressure within the .B chamber will decrease. These effects are additive to increase the loading pressure output: of thexrelay 44, through the pipe 49 to the:.A chamber of-'relay"5fl, resulting in anincrease in pressure within the chamber .A of. relay: 52'and in an opening. ofv the; .feedval've 42.,

The resultant. effect of. steam :flow and steam pressure. (in chamberA of. relay: 50) is-modifiediin accordance with water level as. imposed upon the Bchamber. Likewise, the pressurexefiect in thaA. chamber: of relay .52. is modified bya pressure: inthe B chamber. representativexof water flow rate. Thus whenthe valve 4211s positioned to: satisfy the .dictates of steam pressure, steam flow and; water; level,. it shouldresult ina rate of feed water admissionto the boiler; such as to satisfy said. threevariables. As a check: upon the actual. rate of water flow, the water flow meter-'3] establishes a loading pressure representative ofgwater flowqrate withinthe B chamberofirelay 52, and. compares. this against the. effect established: within the chamber A. If. the.

proper; rate of water admissionis not attained, due perhaps to fluctuations in feed, water pressure then theipositioning of thevalve 4.2 ismodifled; responsive to the interrelation" between the pressure effects within the chambers. A and B of relay 52.

In addition to the. high and low' water level limits described in connectionwith therelay 44, we 1 provide an emergency blowdown. system. including a blowdown valve interposed in the blowdown pipe [3 and under thescontrolof steam pressure through the. agency of a relay 55...

The relay 5 assists the steam pressure controller inmaintaining steam pressure withindesired limits upon a suddden' decrease insteamzdemand. Normally, the blow down valve 54'. is closed. Should a sudden decreasepin steam outflow occur it may. not be possible for the waterv in the. boiler to be immediately evaporated. (or. transferred into surge tank 15 as later described) down to a level correspondingv to the: required steam output of the boiler as a result of'closing down the feedwater supply valve 42 under the dictates of the rising steam, pressure and water level. beyond the standard and the master loading pressure in pipe 26 would also increase above the standard value. When increased sufficiently abovestandard, depending upon the adjustment of "relay 55, the blowdown valve 54 will start to openupso asto assist in dropping the level of water in the boiler drum, and thus decrease the rate of steam generation so asto, bringrthe steam pressure back'to the normal value.

The loading pressurewithin' the: pipe 26 is applied through a branch 26A to both the A. andC chambers of the relay 55,, while the Opposing chamber B is left open to the atmosphere. Thus the loading pressure is doubly effectivein positioning the supply and bleed valves of chamber D of the relay 55. The initial spring loading adjustment of the relay is such that'pressure, within the pipe 26A isnot effective in positioningthe valves of the relay 55 until after aprede'termined The steam pressure would then increase highloading pressure is'attained representativeof a predetermined high steam pressure. For in-- stance the relay may be so adjusted that for an. increase oi. say 25-35 p. s. i..loading pressure from:

normally closed, and is opened only under what.

may be termed an emergency condition ofhigh steam pressure when it is desired to more rapidly relieve water from the boiler drum than is'possible by merely shutting off the supply line l0 and.

waiting for the level to be lowered through evapeoration.

In general, the control system of Figs. 1, 2 an 3 provides that thegreater the depth of immer-- sion the greater the contact area of the boiler water with the heating unit becomes, and consequently the greater the steaming rate of the unit will be. There is, of course, a desirable maximum and minimum level of water in theboiler not to be exceeded.

The steam pressure controller has two principal functions:

1. To control water. level in the boiler and to maintain steam pressure within desired limits at different loads.

2. To open the blowdown valve in the event that the steam pressure increases beyond a predetermined high value. Blowdown' valve 54 is normally-closedand is opened only upon emergency high pressure.

The water level controller 31 functions to open.

the feed water valve 42 as rating (steam flow.) increases, and subsequently as the operatingv water level in the generator is increased. Al-

though called a water level controller, its essential function serves, primarily to maintain an increase in rate of fresh feed water flow as boiler demand increases, and also to stabilize the action of the steam pressure controller in the operation of the feed water valve. v

Water concentration is controlled both by the conductivity controller 2|, operating'on thebleed control valve 20, and by an automatic transferof.

boiler water to and from the surge tank. 15. The operation of such automatic transfer, through the agency of valves ABC-D, wil1 now be :de-

scribed in connection with Figs. 1, 4 and 5.

Referring first to Fig. 1, it will be noted-that: a relatively large pipe lejoins the lower portion of boiler I with the bottom of surge tank 15 for fast transfer of water, from the boiler to the tank, or from the tank to the boiler, under controlof a quick opening solenoid valve C; the valve C being normally closed but is capable of being.

lationship of static pressure in the boilerand; tank, and relationship in static head between the; liquid levels in the boiler and tank. Pressure communication between the two is through'apipe l9 joining the steam space'of boiler-l with the By-passing the larger valve 01 is a to'p o'f tank l and controlled bysolenoid actuated quick opening valve A, which is normally closed when its solenoid is deenergized. The top of tank l5 may be vented to an area of lower pressure through the agency of solenoid actuated quick opening valve B, which is normally closed when its solenoid is deenergized.

The following operations of transfer are ac complished:

' 1. With valves A and C open, valves B and D closed. Pressure is equalized between boiler and tank. The additional static head of water in tank 15 over that of boiler l causes a Fast transfer of Water from tank to boiler (passed through large valve C) 2. With valves A and D open, valves B and C closed. Slow transfer of water from tank to boiler.

i 3. With valves B and C open, valves A and D closed. The tank 15 is vented to atmosphere. The higher pressure in boiler l causes a Fast" transfer of water (passed through valve C) from boiler to tank.

4. With valves B and D open, valves A and C closed. Slow transfer of water from boiler to tank.

5. All valves closed. No transfer in either direction.

The above noted valve combinations; with resulting conditions of water transfer, are desirably accomplished upon predetermined combinations of operating conditions as tabulated in Fig. 4.

The control so far described provides:

1. A control of feed water input rate to vary the electrode immersion with demand, limiting the effect from pressure (both high and low) so that water level will dominate the inflow and not allow an excessive high or low level; responsive to steaming demand (steam pressure and steam flow rate), actual water level (immersion) and with a check on actual, water flow rate against the dictated rate.

2. A control of conductivity of the pool water so that the steaming rate is in direct relation to immersion, first by a bleed from the pool under the control of a conductivity cell, and secondly to counteract the temporary adverse effect upon the conductivity of the pool, such as temperature and concentration dilution of pool by sudden increase in rate of feed upon increasing demand might cause, through supplementing the bleed control with a transfer of water from boiler to tank, or vice versa.

3. Protection against extreme high level in the boiler, by steam pressure opening an emergency blowdown through the agency of a doubling relay.

The present water transfer system supplements previous control by a temporary transfer of water between the tank and boiler, and also to protect against excessive high or low levels in the boiler.

To accomplish operation of the transfer valves we provide electric contacts in the steam pressure controller 23, the boiler water level controller 31 and the tank level controller 4|. Specifically, the Bourdon tube 23, sensitive to steam pressure, positions a contact arm 59 to close circuit with a terminal 60 when the steam pressure reaches a predetermined high pressure, or to close circuit with a terminal 6! should steam pressure reach a predetermined low pressure; beyond which pressures it is not desired operate.

We provide the boiler water level controller 31 with a contact arm 62 arranged to close circuit 12 with a terminal 63 when maximum permissible.

immersion of the electrode is reached; or to close circuit with a terminal 64 when minimum'pe'rmissible immersion of the electrodes is reached;

The tank level controller 41 has a contact arm 65 arranged .to cooperate with three contact terminals 66, 61 and 68. The parts 66 and 61" are of an elongated nature spanning the entire an-' gular travel of the arm 65 except for a non'* contactin gap corresponding. to a liquid level equivalent of about :2 inches across the center of the surge tank. In other words, the actual level of water within the tank l5 may fluctuate, from 2 inches above the tank centerto 2"inches below the tank center without the contact arm' 65 engaging either 66 or 61. shouldtheilevel fall more than 2 inches below the center of the tank, then arm 65 engages 66 and maintains such engagement for all'levels below. On the other, hand, should the level rise 2 inches above sion, (2) value of steam pressure, and (3) the amount of water in the storage tank or conversely the available space within the storage tank to accept water from the boiler.

Considering first the electrodeimmersion, it will be observed from Fig. 4 that three general conditions may exist. l

1. When the water level within the boiler is in what we term the operating range," 1. e. between the points 35, 36, no emergency level condition exists within the boiler and water from the boiler may be transferred to the tank, or may be returned to the boiler from the tank, under the dictates of steam 2. If the level of water within the boiler reaches the maximum immersion limit the arrangement is such that no additional water can be transferred to the boiler from the tank.

3. Should the water level within the boiler reach the minimum immersion limit, then no water may be transferred from the boiler to the tank.

Considering next the effect of steam pressure variations, it will be observed that in general a high steam pressure calls for transferring water from the boiler to the tank so as to decrease the immersion of the electrodes and thereby the steaming possibility. An exception is when the electrodes are already at their minimum immersion limit. If steam pressure is lower than normal, then the tendency is to increase the immersion of the electrodes by transfer of water from the tank to the boiler; except for a condition of where the maximum immersion of the elec-" trodes already exists.

Considering now the level of liquid withinthe tank, as a variable for the purpose of transfer control, it will be observed that four conditions are shown in Fig. 4, namely, the normal desired level at a point i 2 inches across the middle'of the drum, a high or a low operating condition, and a full tank condition. y

The indication of high or low" level within the tank is not a condition of emergency or calling for drastic action. It is quite possible that the liquid'within the tank may be con-' pressure and tank water level.'

sidered as highlevel' andstill-there be available 1 fected were tank 15 filled toan extent that water wouldpass through the valve B.

No-damage toequipment is expected should the tank [5 be emptied, and therefore no emergency empty condition is providedfor.

The particularfunction: of the transfer system just described isin providinga storage place wherein water, at the temperature and concentration of the boiler pool, may be stored for quick return to the boiler whenxneeded. In other words, to take care of sudden, .rapid demand swings or surges upon the boiler steaming .capac-. ity,ithroughm ovinghot concentrated water from the boiler to the tank for storage when not needed, and for returning it to the boiler quickly when needed. Thelatter operation obviates dilution of the water in the vaporizing. pool by sudden infiownof raw feedwater ata relatively lower temperature and relatively'lower value or. conductivity. While the longrange variationin rate of feed input, to accommodate basic changes in rate ofoperation, .is accomplished through regulation, of the feed input valve; 42, as previously described, the transfer system, including the storage tank l5,.acts with a flywheeleffect to take care of rapid variations in demand upon the.

steaming capacity of the'boiler.

Following any surge in :demand,.such as an in crease or decrease in steam demand, with cor responding decrease orincrease insteam pressure, upon the occurrence of; which hot, high concentration water is transferred between the boiler and tank, the valves should be so posi-. tioned as to accomplish a slow return to or from the surge tank to always tend to return the liquid level within the tank to its mid point;

On the premise thatsteaming capacity bears adirect relation to extentof electrodeimm'ersion,

it is apparent that, this is in turn based upon holding the conductivity of the water in the vaporizing pool substantially uniform. Sudden increases in rate of steam outflow will. call for an increased rate of feed water with consequent dilution of the pool water, both as to temperature and conductivity, which effects are adverse to increasing the steam generation rate; Through our invention, however, we tend to avoid such adverse conditions by taking care of sudden surges in demand through transfer of hothighly concentrated water to or from the storage tank.

Immediately following such a surge, with con-.- sequent transfer of water between the boiler and the tank, we so control the transfer valve system as to tend to return the tank level to its mid point in anticipation of another demand upon the tank for water or for storage space. Thus, referring to Figs. 4 and 5, the valvlng arrangement is such as to always tend to return the surge tank level to its mid position.

In the transfer system as illustrated and described, all instrument contacts. are normally o'pen'a'nd the valves A--B-C-D are normally closed when their solenoids- 'A8 --Bs-C 's-Ds are.

deenergized. Desirably, when valve A is open; valve B is closed, and-vice versa. When" valve 0 isopen, for rapid transfer of water, valve D is also open. When/maximum immersionlimit is reached we never transfer water from thetank to the boiler, and conversely, when minimum immersion limit is reached we never transfer water from the boiler to the tank. 'When the water' level within the boiler is within the normal operating range we may 'transfer water either way-' but always follow a demand surge by a transfer tending toreturnthe level within-the surge tank;

to mid-point.

It is not believed necessary to go into detailed" explanationof how -the electric-. circuit of 5 accomplishes the various combinations tabu lated in Fig. 4. It is believed that this is self-a explanatory for the various combinations ofboiler electrode immersion, steam pressure, amitank water level. Suffice it to describe a-slngle set of conditions to'illustrate how the 'circuits of' Fig. 5 will accomplish of Fig. 4; I

Consider operating condition #19 of Fig. 4. where the boiler electrode immersion is 'in its operating range. Steam pressure is at a normal value. Under such conditions-neither the contact arm 62 nor the contact arm '59 is{ engaging a terminal 63, or 60,151 and therespective relays RI, R;2,R3 andR tremain in the J deenergized condition as shown in Fig. 5. 'I-he level of the water within the tank is at the midiZ inch position-and the-contact arm 455-" is not engaging anyof the terminals 66, 61 or 68.

Operation is completely normal in every respect.

in that the boiler electrodeimmersion is satisfactory for the demand and thus the rate 'of va-- porization is such as to maintain uniform-nor- Furthermore, the storage I mal steam pressure. tank It: is approximately one-half full. As seen in Fig. 4, there is no transferof water in either direction through the pipelfi. Valves B, C and D are closed, whilevalve A'isope'ned for pressure equalization between the boiler and-the storage tank. All of the electrical circuits in connection with the four valves are as shown in Fig. 5 with the solenoid-windings Bs, Cs and'Ds deenergized while the winding As is energized.

Assume now that boiler electrode immersion is in the desired operating range, but apparently is slightly higher thanthat corresponding to the rate of steam demand so that steam pressure is-high As illustrated in Fig. 4, unless the storage tank I5 is full (condition #13) there will be a fast transfer of water from the boiler to the tank (conditions 14, 15 or 16) in order to reduce the extent of immersion, and

thus the rate of steam generation, to thereby decrease the high steam pressure to its desir-' able normal'value.

On the other hand, if the boiler electrode immersion is in the desired operating range, but

slightly lower than that corresponding to the boiler,regardless of tank level, to increase elec.-

trode immersion and thereby increase vaporization, and raise steam pressure.

In like'manner, the possible operating condi-a tions may be traced in Fig. 4, to indicate the open or closed positions of *valves A -B-dc-en,

the valve combinations with resultingdirection andspeed'of water transfer between boiler and tank;

While we have illustrated and described certain preferred embodiments of our invention, it will be understood that this-is by way of ex: planation only and not tobe considered" as limiting.

What we claim as new, and desire to secure by Letters Patent of the United States, is:

1. The method of operating a steam generator of the type having two interconnected pressure chambers in one of which electric energy is dissipated in a water pool for vaporizing the water, which includes, controlling the supply of feed water directly to the water pool chamber responsive to a plurality ,of variables in theoperation of the generator, and automatically transferring liquid from one chamber to another when at least one of the variables reaches a predetermined value. I

2. The method of operating a steam generator of, the type having two interconnected pressure chambers in one of which electric energy is dissipated in a water pool for vaporizing the water, which includes, controlling the rate of supply of feed water directly to the water pool chamber conjointly responsive to demand upon the unit and to the amount of water in the pool, and automatically transferring water from one chamber'to another when one of the feed water control variables reaches a predetermined value.

3. The method of controlling the operation of an electric steam. nerator of the type having two interconnected pressure chambers in one of which electric energy is dissipated in a water pool for vaporizing the water, which includes, feeding one-of the chambers with water to form a pool in which the electrodes are immersed for vaporizing the water, determining the extent of immersion, obtaining. a representation of the steam demand upon the unit, continuously utilizing the values of these two variables in controlling the rate of feeding of water to the pool, and transferring water from one chamber to the other when one of the variables reaches a predetermined value.

4. The method of controlling the operation of an electric steam generator of the type having two interconnected pressure chambers in one of which electric energy is dissipated in a water pool for vaporizing the water, which includes, feeding one of the chambers with water to form a pool in which the electrodes are immersed for vaporizing the water, determining the extent of immersion, obtaining a representation of the steam demand upon the unit, continuously determining the rate of supply of feed water to the unit, continuously utilizing the values of these three variables in controlling the rate of feeding of water to the unit, and transferring water from one chamber to another when at least one of the three variables reaches a predetermined value.

5. The method of claim 4 wherein the transfer of water is from the pool chamber to the second chamber when the value of the mentioned variables dictate that water should be removed from the pool, and transferring from the second chamber to the pool chamber when the variables dictate that the pool should be increased.

6. The method of operating an electric steam generator of the type having two interconnected chambers in one of which electric energy is dissipated in a water pool for vaporizing the water and the other chamber providing water storage, which includes, controlling the conductivity of the pool water bya bleed 61: water from the pool responsive to a measure of conductivity, and supplementing such bleed by transferring water from one chamber to the other upon change in unit load.

7. The method of claim 6 wherein the transfer of water is from storage chamber to pool chamber upon sudden increase in demand to temporarily counteract any adverse dilution of pool water such as would be caused by sudden increase in rate of feed supply responsive to increased unit load demand.

8. The method of claim 6 including the further step of protecting the unit against over-immersion of the electrodes through the opening of an emergency blowdown responsive to steam pressure.

9. The method of operating an electric steam generator of the type having two interconnected chambers in one of which electric energy is dis-- sipated in a water pool for vaporizing the water and the other chamber providing water storage, which includes, normally feeding water to the pool chamber in accordance with load demand in a manner such that electrode immersion is within predetermined high and low limits, and supplementing the feed by transferring water from one chamber to another responsive to steam pressure and storage chamber water level.

10. The method of claim 9 including the step of storage chamber responsive to predeterminedhigh steam pressure to thereby decrease the immersion of the electrodes and thus the steaming rate.

13. The method of claim 9 in which normally water is transferred from the storage chamber to the pool chamber responsive to predetermined low steam pressure to thereby increase the immersion of the electrodes and thus the steaming rate.

14. The method of claim 9 in which, following aload change, the water content of the storage chamber is returned toward a predetermined normal value.

15. In combination with anelectric steam generator having two interconnected pressure chambers in one of which are electrodes and having provisions for supplying feed water to the electrode chamber to form a pool in which the electrodes are variably immersed for vaporizing the water, means sensitive to a demand factor on the unit, means'sensitive to. the amount of electrode Y immersion, control means responsive to both said sensitive means adapted to regulate the feed water supplying provisions, and transfermeans arranged to cause transfer of water from one chamber to another when one of saidsensitive means reaches a predetermined value,

16. In combination with an electric steam gen- I erator having two interconnected pressure chambers in one of which are electrodes and hav- I ing provisions for supplying feed water to the electrode chamber to form a pool in which the electrodes are variably immersed for vaporizing the water, first means determining the extent of immersion,- second means determining-isteamdemand upon the unit, third means determining the rate=of supply of feed water,'means continuouslyv controlling the supply. of feed water conjointlyx responsive to said first three named, means, and means adapted to cause a transfer oifwater: from one chamber to another when one of 'the first three named means reaches a predetermined value.

17. In combination with anelectric steam generator having two, interconnected pressure chambers in one of which are electrodes and having provisions fOrSupplying-feed' Water to the electrode chamber to form a poolin which the electrodes are variably immersed for vaporizing the water, means regulating the electrode im-- mersion responsive to demand upon the unit, means regulating bleed of water from the pool responsive to conductivity of the pool water, and meansadapted to cause a transfer of water from onechamber to another responsive to-a variablecondition in the operation of the-unit.

18. In combination with an electric steam generator: having two interconnected pressure chambers in one of which are electrodes and having provisions forsupplying feed water to the electrodechamber to form a pool in which theelectrodes are variably immersedfor'vaporizingthe water, means regulating bleed of water from thepool responsive to conductivity ofthe pool water, and means causing a transfer-of wa-" ter from one chamber to the other upon change in' unit load.

19; In 'combination with an electric steam generator-having two interconnected pressure chambers in one of which are electrodes and having provisions for supplyingfeed water to the-electrode chamber to form a pool in which the electrodes are variably immersed for vapor-- izingthewater, means establishing-a fluidcontrol'pressure representative of demand upon the-- unit, means establishing afluid control-pressurerepresentative of water level within the pool chamber and thereby oftheelectrode immersion, control means adapted to regulate the'rate of supply of water responsive to both said-fluid control pressures, valve means controlling transfer of water-from one chamber-tothe other; and transfer means sensitive to a; selected variablein the operation of theunit adapted-to position said valve means.

2D. In combination-with an electric steamgeneratorhaving two interconnected pressure chambers/in one of which are electrodes'and having provisionsfor supplyingfeed Water to the electrodes chamber to form a-pool in which theelectrodes are variably immersed for vaporizingthe water, means establishing a fluid control pressure continuously representative of electrode immersion, means establishing a second fluid control pressure continuously representative of demandupon the unit, means establishing a third fluid control pressure continuously representative of rate of water supplied to the pool, control means for the water supply to the pool responsive to said-three fluid control pressures, valve; means controlling transfer of water from one chamber. to the other; and aplurality of means separately" responsive; to. steam pressureand electrode immersion and" storage chamber Water level arranged to conjointly control the;

saidvalve means.

21. The method-of operatingan electric steam generator of the type having two interconnected pressure chambers, in. one of. which electric ener'gy is. dissipated in. an waten'pool, for vaporizing: the. water and the: other" chamber pro-'- viding' water storage, which. includes, continuously controlling the extent of immersion of? the electrodes in the pool conjointly responsive. tov

steam demand. upon the unit and to amount of water in; the pool, and continuously controlling;

the conductivity of: the pool water inwhich: the

electrodes are immersed by controllably' bleeding: pool water from the pool chamber and by trans fer-ring water between the pool chamber and the storage chamber.

22; The method of operating anelectric steam generator of the type havingtwo'interconnectedr pressure chambers in one of which electric,

energy is dissipated in a water pool forvaporizin'g the water and the other 'chamber*providing'water storage, which includes, continuously controlling the extent ofimmersion of the electrodes in the pool con'jointly responsive to steam demand, amount of water in the pool, and: rate of feed. water-supplyto the unit; and continuously controlling the conductivity of the pool water in which the electrodes are immersed by controllably bleeding pool water from the pool chamber and by transferring water between the pool chamber and the storage chamber.-

23; The method of operating an electric steam generator of the type having two interconnected pressure chambers in one of which electric energy is dissipated ina waterpool for vaporizing the water and the other chamber providing water storage; which includes, continuously controlling the extent of immersion of theel'ectrodes in the pool conjointly responsive to' steam pressure, rate of steam outflow, extent of electrode immersion, and rateof feed water supply to the unit; and continuously controlling the conductivity of the pool water in which the electrodes are immersed by'controllably bleeding pool water-fromv thepool chamber and by transferring water between the pool chamber and the storage:

chamber;

24'; The method of'operating an electricsteam generator of the type having two interconnected pressure chambers in one of which electric energy is dissipatedin a water pool for vaporizing the water and the other chamber providing Water: storage, which includes, continuously controlling the extent of immersion of'the electrodes in the pool conjointly responsive to a plurality of variables in'theunit'operation, and continuously controlling the conductivity of the pool water; in which the electrodes are immersed by controllably bleeding pool water'from the pool cham-berand by transferring water between the pool chamber and the storage chamber,, the transferring of water from; the storage chamber to the pool chamber being prevented: when predetermined maximum immersion of the electrodes is reached and the transferring of waterfrom the pool chamber to the storage chamber being prevented when predetermined minimum immersion of the electrodes is-reached.

25. Th methodoi'operating'an' electric vapor generator of the type having two interconnected.

pressure chambersin one of'which electric energy isdissipated in-aliquid pool for vaporizing the liquid and theother-chamber providing liquid storage,- Which includes, controlling the specific conductivityof the 'pool liquid by a bleed of liquid from the pool-responsivetoa measure of specific conductivity of the: liquid, and supplementing such bleed-by transferring liquid from one cham-- her, to.;th e otherrupon; change: in .unit loadi 26. In combination with an electric vapor generator having tw interconnected pressure chambers in one of which are electrodes and having provisions for supplying feed liquid to the electrode chamber to form a pool in which the electrodes are variably immersed for vaporizing the liquid, means establishing a fluid control pressure representative of demand upon the unit, means establishing a fluid control pressure representative of liquid level within the pool chamber and thereby of the electrode immersion, control means adapted to regulate the rate of supply of liquid responsive to both said fluid control pressures, valve means controlling transfer of liquid from one chamber to the other, and transfer means sensitive to a selected variable in the operation of the unit adapted to position said valve means.

27. In combination with an electric vapor generator having two interconnected pressure chambers in one of which are electrodes and having provisions for supplying feed liquid to the electrode chamber to form a pool in which the electrodes are variably immersed for vaporizing the liquid, means establishing a fluid control pressure continuously representative of electrode immersion, means establishing a second fluid control pressure continuously representative of demand upon the unit, means establishing a third fluid control pressure continuously representative of rate of liquid supplied to the pool, control means for the liquid supply to-the pool responsive to said three fluid control pressures, valve means controlling transfer of liquid from one chamber to the other, and a plurality of means separately responsive to vapor pressure and electrode immersion and storage chamber liquid level arranged to conjointly control the said valve means.

28. The method of operating a steam generator of the type having two interconnected pressure chambers in one of which electric energy is dissipated in a water pool for vaporizing the water, which includes, controlling the rate of supply of feed water directly to the water pool chamber conjointly responsive to steam outflow rate and to the amount of water in the pool, and automatically transferring water from one chamber to another when one of the feed water control variables reaches a predetermined value.

29. The method of operating an electric steam generator of the type having two interconnected chambers in one of which electric energy is dissipated in a water pool for vaporizing the water and the other chamber providing water storage, which includes, normally feeding water to the pool chamber in accordance with steam outflow rate in a manner such that electrode immersion is within predetermined high and low limits, and supplementing the feed by transferring water from one chamber to another responsive to steam pressure and storage chamber water level.

30. The method of operating an electric steam generator of the type having two interconnected pressure chambers in one of which electric energy is dissipated in a water pool for vaporizing the water and the other chamber providing water storage, which includes, continuously con-- trolling the extent of immersion of the electrodes in the pool conjointly responsive to steam outflow rate from the unit and to amount of water in the pool, and continuously controlling the conductivity of the pool water in which the electrodes are immersed by controllably bleeding pool water from the pool chamber and by transpool chamber and the pressure chambers in one of which electric energy is dissipated in a water pool for vaporizing the water and the other chamber providing water storage, which includes, continuously controlling the extent of immersion of the electrodes in the pool conjointly responsive to steam outflow rate, amount of water in the pool, and rat of feed water supply to the unit; and continuously controlling the conductivity of the pool water in which the electrodes are immersed by controllably bleeding pool water from the pool chamber and by transferring water between the pool chamber and the storage chamber.

32. In combination with an electric steam generator having two interconnected pressure cham bers in one of which are electrodes and having provisions for supplying feed water to the electrode chamber to form a pool in which the electrodes are variably immersed for vaporizing the water, means sensitive to steam outflow rate from the unit, means sensitive to the amount of electrode immersion, control means responsive to both said sensitive means adapted to regulate the feed water supplying provisions, and transfer means arranged to cause transfer of water from one chamber to another when one of said sensitive means reaches a predetermined value.

33. In combination with an electric steam generator having two interconnected pressure chambers in one of which are electrodes and having provisions for supplying feed water to the electrode chamber to form a pool in which the electrodes are variably immersed for vaporizing the water, means establishing a fluid control pres-' sure representative of steam outflow rate from the unit, means establishing a fluid control pressure representative of water level within the pool chamber and thereby of the electrode immersion, control means adapted to regulate the rate of supply of water responsive to both said fluid control pressures, valve means controlling transfer of water from one chamber to the other, and transfer means sensitive to a selected variable in the operation of the unit adapted to position said valve means.

34. In combination with an electric steam generator having two interconnected pressure chambers in one of which are electrodes and having provisions for supplying feed water to the electrode chamber to form a pool in which the electrodes are variably immersed for vaporizing the water, means establishing a fluid control pres sure continuously representative of electrode im mersion, means establishing a second fluid control pressure continuously representative of steam outflow rate from the unit, means establishing a third fluid control pressure continuously representative of rate of water supplied to the pool, control means for the water supply to the pool responsive to said three fluid control pressures, valve means controlling transfer of water from one chamber to the other, and a plurality of means separately responsive to steam pressure and electrode immersion and storage water level arranged to conjointly control the said valve means.

HARVEY C. MITTENDORF. WILLIAM L. PAULISON, JR.

(References on following pa REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 5 Number Name Date 1,597,362 Henriksson Aug. 24, 1926 1,665,793 Sandborgh Apr. 10, 1928 1,902,842 Eaton Mar. 28, 1933 1,975,086 Dickey Oct. 2, 1934 10 Number 22 N Name Date Eaton July 2, 1935' Dickey Nov. 9, 1937 Eaton Jan. 2, 1940 Junkins Sept. 1, 1942 Eaton Nov. 9, 1948 Eaton -2 Nov. 9, 1948 Mittendorf Oct. 25, 1949 Birchler et a1. Oct. 25, 1949 

